Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

ATP PRODUCTION IN MITOCHONDRIA
781
H +
H +
flagellum
stator
proteins
rotor
proteins
H +
EXTRACELLULAR SPACE
H + H +
proton pump
outer bacterial
membrane
peptidoglycan
layer
inner bacterial
membrane
(plasma
membrane)
Figure 14–35 The rotation of the
bacterial flagellum driven by H + flow.
The flagellum is attached to a series of
protein rings (pink), which are embedded in
the outer and inner membranes and rotate
with the flagellum. The rotation is driven
by a flow of protons through an outer ring
of proteins (the stator) by mechanisms
that may resemble those used by the atp
synthase. However, the flow of protons
in the flagellar motor is always toward
the cytosol, both during clockwise and
counterclockwise rotation, whereas in
ATP synthase this flow reverses with the
direction of rotation (Movie 14.8).
flagellar motor rotating at more
than 100 revolutions per second
CYTOPLASM
bacteria, the ATP synthase works in reverse, using the ATP produced by glycolysis
to pump H + and establish a proton gradient across the plasma membrane.
Bacteria, including MBoC6 the m14.17/14.35
strict anaerobes, maintain a proton gradient across
their plasma membrane that is harnessed to drive many other processes. It can
be used to drive a flagellar motor, for example (Figure 14–35). This gradient is
harnessed to pump Na + out of the bacterium via a Na + –H + antiporter that takes
the place of the Na + -K + pump of eukaryotic cells. The gradient is also used for the
active inward transport of nutrients, such as most amino acids and many sugars:
each nutrient is dragged into the cell along with one or more protons through a
specific symporter (Figure 14–36; see also Chapter 11). In animal cells, by contrast,
most inward transport across the plasma membrane is driven by the Na +
gradient (high Na + outside, low Na + inside) that is established by the Na + -K + pump
(see Figure 11–15).
Some unusual bacteria have adapted to live in a very alkaline environment
and yet must maintain their cytoplasm at a physiological pH. For these cells, any
attempt to generate an electrochemical H + gradient would be opposed by a large
H + concentration gradient in the wrong direction (H + higher inside than outside).
Presumably for this reason, some of these bacteria substitute Na + for H + in all of
their chemiosmotic mechanisms. The respiratory chain pumps Na + out of the
cell, the transport systems and flagellar motor are driven by an inward flux of Na + ,
and a Na + -driven ATP synthase synthesizes ATP. The existence of such bacteria
demonstrates a critical point: the principle of chemiosmosis is more fundamental
than the proton-motive force on which it is normally based.
As we discuss next, an ATP synthase coupled to chemiosmotic processes is
also a central feature of plants, where it plays critical roles in both mitochondria
and chloroplasts.
Figure 14–36 The importance of H + -driven transport in bacteria.
A proton-motive force generated across the plasma membrane pumps
nutrients into the cell and expels Na + . (A) In an aerobic bacterium,
a respiratory chain fed by the oxidation of substrates produces an
electrochemical proton gradient across the plasma membrane. This gradient
is then harnessed to make atp, as well as to transport nutrients (proline,
succinate, lactose, and lysine) into the cell and to pump Na + out of the cell.
(B) When the same bacterium grows under anaerobic conditions, it derives its
ATP from glycolysis. As indicated, the atp synthase in the plasma membrane
then hydrolyzes some of this atp to establish an electrochemical proton
gradient that drives the same transport processes that depend on respiratory
chain proton-pumping in (A).
(A) AEROBIC CONDITIONS
H +
H + ADP
+ P i
e –
ATP
H +
H +
lactose
proline
H + CYTOPLASM
H +
lysine
lactose
respiratory chain
Na +
CYTOPLASM
H +
lysine
Na +
H +
ATP synthase
succinate
bacterial
plasma
membrane
H +
(B) ANAEROBIC CONDITIONS
H +
ATP
P i + ADP
H +
H +
proline
H +
succinate